To print an image on an appropriate substrate, a typical “electrostatic” digital printer first forms a copy of the image, conventionally referred to as a “latent image”, on a photosensitive surface of a cylindrical roller, hereinafter referred to as a “photosensitive imaging plate” (PIP). To form the latent image a charger deposits a substantially uniform charge density on the photosurface. A laser then scans the charged photosensitive surface and discharges regions thereon to generate a pattern of charged and discharged, or partially discharged, pixels on the photosurface that forms the latent image and replicates the image to be printed.
A developer electrode is used to develop the latent image by applying ink or toner, hereinafter generically, toner, to the photosensitive surface. The toner comprises charged toner particles of a desired color, which adhere to the charged or the discharged or partially discharged pixels, depending on the details of the printing process used. The toner on the PIP is then optionally transferred from the PIP to a suitable transfer surface of another roller, conventionally referred to as an “intermediate transfer member” (ITM). The toner is transferred from the ITM (or the PIP, if no ITM is used) to the substrate, to print the image when the substrate passes through a nip between the ITM and an impression roller. In some cases the toner is transferred directly from the PIP to the substrate without an intermediate transfer via the ITM.
In many printing processes the toner is a liquid toner comprising charged toner particles dispersed in a non-polar carrier liquid having a relatively high electrical resistance. The toner particles generally comprise a polymer in which, optionally, a pigment is dispersed. In order for the toner particles to be properly electrophoretically transferred and adhered to the PIP, and optionally to the ITM or to the substrate, the toner particles are charged with an amount of charge that endows them with an appropriate mobility. Charging is generally accomplished by adding to the particles at least one charge control agent, alternatively referred to as a “charge director” (CD), which usually comprises organic salt or zwitterionic molecules.
A charge priming agent, conventionally referred to as a charge adjuvant (CA), that promotes charging of the toner particles by the charge director is also often added to the particles. For liquid toners, aluminum alkoxide, or a metallic soap, for example, magnesium stearate, aluminum tristearate or octoate, are commonly used as a charge adjuvant.
Some of the commonly used CAs, such as aluminum or magnesium stearate, are relatively insoluble at room temperature in carrier liquids typically used for liquid toners. To provide for effective activity of these CAs, they are generally added to a toner during production when toner ingredients are ground together so that the grinding process can thoroughly mix the CAs with the toner ingredients and optionally, react with them. However, because these CAs must in general be ground together with toner ingredients, they cannot usually be added to a toner after production to adjust for deviation of toner particle charge from a desired normative charge or to reverse or moderate dissipation of toner charge that may occur during the shelf life of the toner.
Some CAs, such as the aluminum alkoxides (which are the reaction product of a metal and an appropriate alcohol), are practically soluble in toner carrier liquids. However, these CAs tend to be highly reactive not only with toner ingredients but also with impurities, in particular water, that typically adulterate liquid toners. They tend to generate charge not only on toner particles, but to add to “parasitic” charge in the carrier liquid by interacting with the charge director and molecules in the carrier liquid that are not associated with the toner particles to generate charged molecular species. The parasitic charge contributes to background “parasitic conductivity” of the toner that tends to degrade quality of an image printed with the toner. In addition, the particular affinity of these CAs to water often results in their undergoing hydrolysis and precipitating out of a toner dispersion to which they are added. As a result, it is generally difficult to provide consistent and controllable levels of toner particle charging with these CAs.
U.S. Pat. Nos. 4,794,651 and 5,565,299 describe materials and processes for preparing a liquid toner. U.S. Pat. Nos. 4,707,429 and 5,225,306 describe materials and processes for preparing a liquid toner using an aluminum tri-stearate charge adjuvant and aluminum alkoxide charge priming agent (Isopar soluble CA). U.S. Pat. No. 5,573,882 describes materials and processes for preparing a liquid toner using a charge adjuvant. U.S. Pat. No. 5,393,635 describes a negative charge director for liquid electrographic toners in which a negative charge is generated by a weakly associating charged functional group covalently bonded to the resin of the toner particles and a very strongly chelating molecule dispersed in the liquid phase to achieve charge separation. The disclosures of all the aforementioned U.S. patents are incorporated herein by reference.